This invention relates to friction materials and their manufacture, and more particularly to friction materials of the kind which are for dry running and which are molded.
Molded clutch facings are usually manufactured by subjecting an annular shaped preform containing a binder, reinforcing fibers, fillers and friction modifiers to the action of heat and pressure in a suitably shaped die. The binder, normally a thermosetting resin or vulcanizable elastomer or mixtures thereof, softens and flows to contact the reinforcing fibers and particles of filler or friction modifier then hardens under the continued application of heat and pressure. Alternatively, the preform may consist of a yarn or tape impregnated with a mixture of binders, fillers and friction modifiers and wound in a spiral or undulating pattern to produce an annular shape. The facings are usually die cured for periods of typically 4 minutes to 15 minutes and then may be further heated at temperatures of 150.degree. C.-250.degree. C. to complete the cure of the binder and also to improve friction properties.
Conventional friction materials molded from a mixture of asbestos fibers, thermosetting resins or vulcanizable elastomers or mixtures thereof and particulate fillers and friction modifiers are not normally consolidated to achieve the theoretical densities of the mixtures, i.e., they contain proportions of voids, even when cured at temperatures of up to 160.degree. C. and pressures of up to 3 tons per square inch. The presence of voids, especially when interconnected to produce a permeable structure, is desirable to produce materials with good resistance to "fade", i.e., reduction in coefficient of friction when friction materials operate at high temperatures. Molded clutch facings comprising asbestos, a phenolformaldehyde resin binder, particulate fillers and friction modifiers are manufactured with densities typically in the range of 85-95% of theoretical density corresponding to void volumes of 15-5%.
When replacing asbestos fibers by fibers such as cellulose, glass, mineral wools (e.g., basalt or slagwools), alumino-silicate, polyacrylonitrile, polyamides, aromatic polyamides and carbon fibers, I have found that conventionally press cured materials are consolidated to greater percentages of their theoretical densities than are asbestos-based materials. This results in impermeable structures with very low void volumes. Such non-asbestos friction materials may possess good physical strengths but have poor resistance to fade and wear. Some improvement in fade resistance may be made by moulding or machining grooves into the working face of the friction material, but such improvements are not sufficient to match the performance of the best asbestos-based friction materials.
I have found that non-asbestos friction materials may be manufactured with high permeability with all its attendant advantages, and that the wear and fade properties of clutch facing may be remarkably improved thereby.